In roller mills, which are also known as air-swept roller mills or roller bowl mills, a grinding material flow supplied to the centre of a grinding bowl and containing fresh material and coarse particles from a classifier, which is generally integrated into the roller mill, is moved by centrifugal force to the edge of the grinding bowl and to a blade ring surrounding the latter. The centrifugal force acting on the grinding material is in particular dependent on the diameter and speed of the grinding bowl.
The throughput of a roller mill is essentially determined by the available grinding surface, i.e. the number and size of the grinding rollers used and the grinding speed. This assumes an adequate supply of grinding material to each roller.
In principle, by a speed increase the throughput of a roller mill can be increased for the same, predetermined geometries and the same number of grinding rollers. Consequently a smaller roller mill can be used, which leads to lower costs.
However, a speed increase is simultaneously associated with a centrifugal force increase, which leads to the grinding bowl being partly or completely emptied in the area between two successive grinding rollers, So that the latter are no longer supplied with grinding material and the sought throughput increase cannot be achieved.
DE 36 42 814 A1 discloses a roller mill, which is provided with a retaining device between two grinding rollers for controlling the grinding material movement in the vicinity and on the grinding path. The retaining device is constructed as an arcuate baffle or deflector, which is fixed by means of holding devices to the roller mill housing and is adjustable as regards height and inclination.
A roller mill described in German patent 1 507 579 has facing a material feed and between two grinding rollers a deflector, through which it is intended to bring about a classification of the grinding material moved in the direction of the bowl edge. The shape and height of the deflector bring about a splitting up of the grinding material and a separate carrying off of the fine material flow and a retention of the coarse material on the grinding path.
The disadvantages of these known means are a relatively high friction between the grinding material and the deflector used as retaining device, which leads to an increased drive power requirement. As a result of the friction increased wear takes place to the deflectors and also the holding devices, which requires corresponding maintenance measures and hard surfacing of the deflectors and holding devices.
DE 39 21 986 C1 discloses a roller mill with precompacting or smoothing rollers. These precompacting or smoothing rollers are provided for equalization and levelling of the grinding bed and are in each case positioned upstream of a grinding roller. The smoothing or precompacting rollers are not constructed as retention devices for the grinding material moved outwards on the grinding path of the grinding bowl, but instead for venting and smoothing of the grinding material to be supplied to a grinding roller and in particular for eliminating a "bow wave" of an air-dust mixture. For obtaining this sought effect a precompacting or smoothing roller is positioned upstream of a grinding roller. However, this document does not disclose constructing and arranging the precompacting or smoothing rollers for retaining the grinding material on the grinding path. Thus, a retention effect of the smoothing or precompacting roller is only possible to a limited extent over the end face of such a roller.
The object of the invention is to provide a roller mill with a low-wear, and in particular efficient retaining device, which ensures an adequate grinding material supply to the grinding rollers, both at normal and increased grinding material speed, so that with reduced effort and expenditure a higher throughput is ensured.
According to the invention, this object is achieved by the features of claim 1. Appropriate and advantageous developments appear in the subclaims and description relative to the drawings.
The invention is based on the idea of using the particularly low friction of precompacting or smoothing rollers, which is clearly reduced compared with the known deflectors, for the sought and in particular efficient retention and to construct the precompacting or smoothing rollers as damming rolls with a predeterminable damming effect and positioning same between in each case two grinding rollers for a weir-like retention of the grinding material.
According to the invention, the roller mill has a rotary damming surface wall between two grinding rollers as a retaining device. On the grinding material moved outwards on the grinding path of the grinding bowl said rotary damming surface wall exerts a damming action. According to the invention, the damming action is brought about by a rotary damming surface wall with at least one damming roll, which is adjacent between two grinding rollers and which has at least one further damming surface besides a damming surface which is constructed on an inner end face facing the rotation axis of the grinding bowl. According to the invention, a damming roll has at least one damming ring on which is formed the further damming surface.
It is also possible to provide a damming roll with more than one further damming surface, in that several damming rings are located on a damming roll. The additional damming surfaces are then constructed on the insides of the damming rings, said damming rings appropriately projecting radially from the circumferential surface of a damming roll.
Appropriately the damming faces of the damming rings almost form a right angle with the rotation axis or the circumferential surface of the damming roll.
As a result of said further damming faces the retaining action of a damming roll is further increased. Grinding material, which as a result of the centrifugal action is moved from the grinding path in the direction of the damming rim of the grinding bowl, said movement taking place on a spiral path, is firstly blocked by the damming face of the inside end face of the damming roll and is subsequently retained on one or more further damming faces of the damming rings. Therefore the damming rings act as a weir or a blocking device and through the rotary construction of the damming wall an extremely low friction occurs.
In a particularly advantageous construction, beside an inner damming face, a damming roll has in the vicinity of the inner end face at least on the outer end face directed towards the roller mill casing wall, a damming ring, which forms an outer weir or an outer damming face.
The damming rings can appropriately be subsequently fitted to the damming rolls. A detachable arrangement permits an adaptation of the damming faces of the damming rings in accordance with the particular needs.
Appropriately the damming rings are made from a wear-resistant material and roll on the grinding bed with a circumferential surface with a relatively small, radial width.
In an alternative embodiment, a roller mill has as a retaining device a rotary damming surface wall formed from several, e.g. two damming rolls between in each case two grinding rollers. The several damming rolls are so positioned and dimensioned that their end faces form a rotary weir against the outflow of grinding material.
It is particularly advantageous that the rotary damming surface wall is formed by damming rolls rolling on the grinding bed and whose end face are arranged adjacent to one another and with a very small spacing with respect to the adjacent grinding roller.
In principle the damming rolls have no grinding action and are consequently generally exposed to little or no force, so that they do not "jump". Therefore, for a very good damming action, the friction is kept extremely low, which has an advantageous influence on the energy balance.
For the intended damming action or for the action of a weir against an outflow of the grinding material with a particularly low friction, it is appropriate to construct the damming rolls with a very low weight. The damming roll requires no defined additional force besides its own weight, because the damming roll performs no grinding. Appropriately a spring damping system is provided with which the damming roll is held on the grinding bed.
If there is a holding down force, e.g. by a spring damping system, a jumping of the damming rolls can be avoided.
It is avantageous to limit the free mobility of the damming rolls in the downwards direction towards the grinding path of the grinding bowl with the aid of a mechanism. A particularly simple limiting mechanism has a lever with a stop, a setscrew or a buffer.
In order to provide a very tight weir or a tight damming face wall, the damming rolls are so positioned that their rotation axes are under a leading angle or a following/trailing angle with respect to a radial arrangement.
As a function of the construction of the damming rolls, e.g. as a conical damming roll, cylindrical or also cambered damming roll, the damming rolls are so positioned that their extended rotation axes form an intersection with the rotation axis of the grinding bowl or with the longitudinal axis of the roller mill.
As a function of the mill geometry, the grinding material and the desired throughput, the damming rolls can be positioned in such a way that the damming faces of the inner end faces have a different radial spacing and/or a different inclination with respect to the rotation axis of the grinding bowl or to the longitudinal axis of the roller mill and overlap one another and/or the grinding rollers.
Alternatively to a purely rolling movement or with a rolling movement superimposed on a sliding movement, as a function of the arrangement of the damming rolls and the intersections of the rotation axes with the rotation axes of the grinding bowl above or below the grinding path plane, the damming roll can be driven in rotary manner in addition to the bowl. In addition, the circumferential surface areas of the damming roll formed between the damming rings can carry out a levelling or also a smoothing and precompression of the grinding bed, if the damming roll has a corresponding weight or is subject to additional force action.
The advantages of the roller mill according to the invention with the damming rolls as retaining devices are an extremely effective grinding material retention between the grinding rollers. As the grinding material is prevented from passing over the bowl rim and into the vicinity of the blade ring, a disadvantageous loading of the classifying air flow with inadequately comminuted grinding material is avoided. This relieves the mill circuit and a reduced delivery work is required, which improves the energy balance.